Which separation technique is used to separate helium from a mixture of helium and methane

Which of these is a physical property

16. in a reaction that has reached equilibrium, a. the forward and reverse reactions are occurring at the same rate. b. the reactants and products are in equal concentrations. c. the forward reaction has gone further than the reverse reaction. d. there are equal numbers of atoms on both sides of the equation. e. a, b, and d are correct.

Why do electrons further from the nucleus have more energy